Metallic articles may be made with a directionally solidified grain structure to enhance their mechanical properties at elevated temperatures. In directional solidification, molten metal in a mold defining the shape of the article is cooled unidirectionally from one end of the mold. The metal solidifies first at the end from which heat is removed and then along the length of the mold as the temperature falls below the solidus temperature. The resulting structure has a number of grains that are elongated along the length of the mold parallel to the heat flow direction. The grain boundaries are parallel to the heat flow direction as well. The grains typically exhibit an oriented grain structure according to the fastest growing crystallographic direction or a seeded orientation introduced at the end first solidified. The grain orientation is selected to achieve good high temperature properties.
In service, the article made by directional solidification is positioned such that the major mechanical loading is applied parallel to the heat flow direction during solidification. The orientation of the grain structure parallel to the heat flow direction places the greatest material strength in this direction. Additionally, the orientation of the grain boundaries parallel to the heat flow direction reduces the incidence of grain boundary creep. Directional solidification is used to fabricate cast articles of nickel-base superalloys to be used in the hottest portions of aircraft gas turbine engines.
When the article is directionally solidified, there may be casting defects, both of types common to all casting processes and also of types unique to directional solidification. These defects are often manifested as cracks, particularly intergranular cracks, that extend parallel to the direction of the solidification. There may be other types of defects produced during solidification and also during service.
The directionally solidified articles are relatively expensive to produce. It is therefore desirable to repair the defects produced during casting or service, if such repair is feasible. In one approach, the defect may be repaired by a welding process, in which the defect is filled with a filler metal and solidified, or by other filling procedures. Such techniques are known for equiaxed articles. However, when applied to directionally solidified articles, the result is an inadequate repair that has an inhomogeneous microstructure and whose mechanical properties are unacceptably low. The repaired article may also tend to be of less ductility than the defect-free article.
There is a need for an improved approach to the repair of directionally solidified metallic articles. The present invention fulfills this need, and further provides related advantages.
The present invention provides a method for repairing directionally solidified articles. This approach produces a chemically homogeneous structure, and a grain structure that is oriented similarly to that of the remainder of the article. The result is that the repaired article has properties which are the same as or quite close to those of the defect-free portion of the article. The repaired article may therefore be used in service without substantial reduction in properties as compared with a defect-free article.
A method for repairing a directionally solidified article comprises the steps of furnishing a directionally solidified article comprising a base material having a solidus temperature and having a repair region with a grain structure of grains elongated substantially parallel to a solidification direction. The repair region includes a defect that is elongated parallel to the solidification direction. The article is heated to a repair temperature of from about 60 to about 98 percent of the solidus temperature, preferably from about 60 to about 80 percent of the solidus temperature, of the base material in a chamber containing a protective gas that inhibits oxidation of the base material. The defect is filled with a filler metal while maintaining the article at the repair temperature. The step of filling includes the steps of providing a source of a filler metal of substantially the same composition as the base material of the directionally solidified article, and melting the filler metal into the defect progressively while moving the source of the filler metal relative to the article in a direction parallel to the solidification direction, so that the filler metal solidifies within the defect. Optionally, heat may be artificially extracted from the article in a heat-flow direction that is within about 45 degrees of the solidification direction.
The article is preferably made of a nickel base superalloy. The defect in the article is typically a crack extending parallel to the solidification direction, and more typically is an intergranular crack. The invention is also operable to repair other types of defects as well. Prior to heating, it is preferred to remove any foreign matter present in the defect. The removal of the foreign matter is usually accomplished by grinding away base material around the defect, creating a cavity that is to be filled with the filler metal, and chemically cleaning the repair region containing the defect.
The present approach produces a repaired region in which the original defect is filled with the same material as the base metal of the article. The heating of the article to a high temperature during the filling of the defect reduces the incidence of incompatibility between the filler metal and the base metal, and also reduces the likelihood of failures of the base metal due to low ductility of the base metal in intermediate temperature ranges.
The grain structure of the repaired region that originally contained the defect is similar to that of the rest of the article. The grain structure of the repaired region has grains of the base-metal composition oriented parallel to the original heat flow direction, and the grain boundaries are also parallel to the heat flow direction. The grain size may be different, but the oriented grain structure of the repaired region results in no impairment of properties as might be observed if the grain structure in the repaired region were equiaxed or had grain boundaries perpendicular to the original heat flow direction.
The present approach thus allows defects in as-cast or service-utilized directionally solidified articles to be repaired with very little, if any, reduction in the properties of the articles. Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.